In a conventional microwave oven as shown in FIG. 1, microwave oscillated by a magnetron 1 is fed to a heating chamber 4 provided in a oven body 3' through a wave guide 2, whereby a food 6 laid on a turn table 5 can be heated by microwave. A temperature sensor of an infrared temperature sensor 7 is provided in the exterior of the heating chamber for sensing the temperature of the food 6 by receiving infrared rays radiated from the food 6, whereby the output of the magnetron is controlled on the basis of the temperature signal produced by the sensor 7.
In general, the aforementioned microwave ovens are so arranged as to prevent leakage of microwaves out of the heating chamber. However, it has been very difficult to completely suppress leakage of the microwave out of the chamber, and therefore, in the conventional microwave ovens the output of the temperature sensor is accompanied by noise due to the leaked microwave.
More specifically, for example, where such leakage of the microwave occurs, one example of the output signal of the sensor 7 is depicted in FIG. 5 from which it is clearly seen that the temperature signal is accompanied by the noise, and the output of the sensor 7 increases as the cooking time lapses with remarkable fluctuation of the output value due to the noise. When such signal including the noise as shown in FIG. 2 is used as the temperature control signal of the microwave oven, assuming that the desired cooking temperature is T1, if the sensor 7 produces a temperature signal of T1 which is increased by the noise despite that an actual temperature of the food in the oven is T2 which is lower than the desired temperature T1, the microwave oven stops heating at the time A with the actual food temperature T2, then the food may be badly finished.
As mentioned above, the conventional microwave oven has a disadvantage that the S/N ratio of the output signal of the sensor 7 is lowered by the noise and a correct temperature control of the food in the microwave oven can not be performed by the noise and a good finish of cooking can not be expected.
Furthermore, in the microwave oven, there tends to occur the temperature difference between the surface and the inner portion of the food, i.e., the surface of the food is at the desired temperature but the inner portion of the food is lower than the desired temperature. If the microwave heating is finished under such state, a good finish of the cooking can not be made since the temperature of the food as a whole may be lower than the desired temperature.
Japanese patent publication NO. 12698/1978 discloses a way of detecting the temperature of the food for used in a microwave oven in which the temperature of the food is detected during stoppage of the magnetron. Also Japanese patent publication (unexamined) No. 9359/1980 discloses a microwave oven in which the temperature of the food is repeatedly compared with a predetermined value a plurality of times, whereby the microwave heating is stopped. However, both of the prior arts do not solve the above mentioned problems.
In attempting to prevent such incomplete finishing of the cooking mentioned as above by merely exciting the magnetron again after short interruption of the magnetron until the entire food is heated up to the desired temperature, excitation and interruption of the magnetron may be undesirably frequently repeated.